##Al_2(SO_4)_3##

The sulfate anion has a ##2-## charge, whereas the aluminum cation has a ##3+## charge. The ionic compound must be electrically neutral. Thus we take 2 aluminum ions (6+ charge) and 3 sulfate ions (6-) charge, and achieve electrical neutrality. Are you happy with this treatment? What is the formula for sodium sulfate?

##Al_2(SO_4)_3##

The sulfate anion has a ##2-## charge, whereas the aluminum cation has a ##3+## charge. The ionic compound must be electrically neutral. Thus we take 2 aluminum ions (6+ charge) and 3 sulfate ions (6-) charge, and achieve electrical neutrality. Are you happy with this treatment? What is the formula for sodium sulfate?